1. Field of the Invention
The present invention relates to an improvement in a wire electric discharge machine, and particularly to a controller of a wire electric discharge machine capable of monitoring a change in thickness of a workpiece and a change in machining current density and also improving a machining performance on a workpiece having a change in thickness.
2. Description of the Related Art
FIG. 28 shows an outline of a conventional wire electric discharge machine. A main pulse generator 1 is constituted by a direct current power source, a circuit comprising switching elements such as transistors and a circuit charging and discharging a capacitor for applying voltage to a gap between a wire electrode 4 and a workpiece 5 to carry out electric discharge machining. A detection voltage generator 2 is constituted by a circuit comprising active elements such as transistors, resistors and capacitors and a direct current power source for applying pulse voltage (voltage lower than main pulse voltage) between the wire electrode 4 and the workpiece 5 for detecting whether the gap between the wire electrode 4 and the workpiece 5 is dischargeable.
Conductive brushes 3 are for conducting electricity to the wire electrode 4 and connected to terminals of the main pulse generator 1 and the detection voltage generator 2 on one side. Further, the workpiece 5 is connected to terminals of the main pulse generator 1 and the detection voltage generator 2 on other side and pulse voltage is applied from the main pulse generator 1 or the detection voltage generator 2 between the traveling wire electrode 4 and the workpiece 5.
A discharge gap detecting device 6 is connected to the workpiece 5 and the wire electrode 4, determines whether discharge gap is brought into a dischargeable state based on lowering of the detection pulse voltage and outputs a signal for servo feed to a feed pulse calculating device 7 by the detected voltage changed by a change in the discharge gap. The feed pulse calculating device 7 produces a series of pulses a feed pulse interval of which is normally controlled such that gap average voltage becomes constant to optimize repetition of discharge based on the signal for servo feed and outputs the series of pulses to a feed pulse distributing device 8. The feed pulse distributing device 8 distributes the series of pulses to the drive pulses of X-axis and Y-axis based on a machining program and outputs the drive pulses to an X-axis motor control device 9 and a Y-axis motor control device 10 for driving a table mounted with the workpiece 5.
Further, according to a conventional wire electric discharge machine shown by FIG. 29, a current detecting circuit 11 detects main pulse current and outputs an average machining current value in a predetermined time period. A display device 12 displays the average machining current outputted from the current detecting circuit 11, average machining voltage outputted from the discharge gap detecting device 6 and feed speed outputted from the feed pulse calculating device 7, respectively, by receiving data of numeral values or levels.
First, in order to detect whether electricity can be discharged between the workpiece 5 and the wire electrode 4, detection pulse voltage is generated from the detection voltage generator 2 and is applied to the gap between the workpiece 5 and the wire electrode 4. When electricity conduction is caused between the workpiece 5 and the wire electrode 4 and voltage drop is caused between the workpiece 5 and the wire electrode 4, the discharge gap detecting device 6 detects the voltage drop, determines that electricity can be discharged, transmits a main pulse applying signal to the main pulse generator 1 to thereby generate a main pulse from the main pulse generator 1 and flows main pulse current (discharge machining current) to the gap between the workpiece 5 and the wire electrode 4. Thereafter, the discharge gap detecting device 6 applies again a detection pulse to the gap after elapse of a pertinent pause time period for cooling the gap. The discharge machining is carried out by repeatedly executing the operational cycle.
In respect of a situation of repeated discharge the feed pulse calculating device 7 produces a series of pulses the feed pulse interval of which is normally controlled such that average voltage of the gap becomes constant in order to optimize repetition of discharge at the gap by the discharge gap detecting device 6 and the feed pulse calculating device 7. The feed pulse distributing device 8 distributes the series of pulses into drive pulses of X-axis and Y-axis based on a machining program, outputs the drive pulses respectively to the X-axis motor control device 9 and the Y-axis motor control device 10, drives a table mounted with the workpiece 5 and carries out machining instructed by the machining program on the workpiece 5.
Further, according to the conventional wire electric discharge machine shown by FIG. 29, the average machining current detected by the current detecting circuit 11, the average machining voltage detected by the discharge gap detecting device 6 and the feed speed calculated by the feed pulse calculating device 7, are displayed on the display device 12 to thereby indicate a state in machining.
FIG. 11 shows monitor waveforms of the average machining voltage and the average machining current when the workpiece 5 having a section shown by FIG. 10 is sliced by a conventional controller of a wire electric discharge machine mentioned above. Further, FIG. 23 shows monitor waveforms of the average machining voltage and the average machining current when the workpiece 5 having a section shown by FIG. 22 is sliced by the conventional controller of a wire electric discharge machine mentioned above.
Although the plate thickness of the workpiece 5 is varied, as shown by FIG. 11 and FIG. 23, in respect of a change in the plate thickness, the average machining voltage remains substantially constant as a whole. Similarly, the average machining current also remains substantially constant.
Normally, when the plate thickness of the workpiece 5 is changed, disconnection of the wire electrode 4 is frequently caused immediately after machining is particularly shifted from a thick portion to a thin portion. Cause therefor seems to be that pulse current is liable to concentrate at one location at a portion having a thin plate thickness. Therefore, conventionally, in order to avoid pulse current from being concentrated on a portion having a thin plate thickness, machining is carried out by modifying a machining condition to that in conformity with the portion having a thin plate thickness from start of machining such that proper average machining current is produced. This operation gives rise to a considerable drop in the machining speed.
When a change in the plate thickness is previously known and when, for example, an operation of reducing machining current in the case of a thin plate thickness and increasing the current in the case of a thick plate thickness can be carried out, the machining time period can be shortened by that amount. Conventionally, as a method of adjusting machining current by finding a change in a plate thickness, there has been devised a method in which thickness information is read from the drawings, a machining program is fabricated by including information of increasing or decreasing machining current and the information is instructed or displayed in machining. However, the thickness information is not necessarily constituted ordinarily to directly provide from dimensions of the drawings. Therefore, in constituting a machining program, the thickness information needs to particularly calculate and the thickness information needs to instruct previously. Otherwise, in carrying out cutting as in a cut model or wire electric discharge machining after coarsely constituting a workpiece already by machining, although the thickness information may particularly be calculated and instructed previously, such an operation per se is very difficult.
Discharge in electric discharge machining is started by searching a very small conductive path such that a gap formed between an electrode and a workpiece opposed thereto becomes several tens .mu.m or less by a detection pulse or the like, thereafter flowing main pulse current and vaporizing or melting to scatter the very small conductive path or very small portions of the electrode and the workpiece in contact therewith forcibly by thermal energy produced there. Further, a series of the discharge machining cycle is finished by pause of current and a cooling action of a machining fluid.
The degree of evaporation or melting to scatter at the both very small portions is determined by the magnitude of a peak value of the main pulse current having steep rise, thermally related properties of materials of the electrode and the workpiece such as melting heat or thermal conductivity and machining environmental properties such as properties related to cooling of an insulating fluid such as latent heat of evaporation or viscosity.
When the plate thickness of the workpiece mentioned above is thin, a time period until generation of successive discharge at a vicinity of a discharge portion is shorter than that when the plate thickness is thick and accordingly, successive main pulse current is applied before sufficiently cooling the portion. Accordingly, depending on the machining environmental properties mentioned above, heat is concentrated while the discharge portion remains uncooled. Further, there emerges a state in which a molten state still remains when successive main pulse is applied. In such a situation, the very small portions of the electrode and the workpiece cannot be evaporated or melted to scatter, the machining efficiency is extremely deteriorated and a state in which the discharge machining can no more be carried out is brought about. When the main pulse is applied further even under the situation, the wire constituting the electrode is heated and destructed and finally reaches disconnection by being unable to withstand the tensile strength of the wire when the wire is travelling.
Meanwhile, when the plate thickness differs, a number of repetition of discharge caused in progressing a constant distance also differs. Therefore, an effect of enlarging the gap between the wire electrode and the workpiece caused by secondary discharge via sludge becomes significant to thereby cause a problem in which an amount of enlarging in machining portions of the workpiece having different plate thicknesses is not constant.
In this way, there poses a problem in which when the machining speed in intended to increase, there is a limit in energy capable of being applied to machining by an environment of machining such as a material or a plate thickness of the workpiece and the amount of enlarging machining at portions of the workpiece having different plate thicknesses is not constant as a result by a difference in a ratio of applied energy to plate thickness.
As mentioned above, in order to avoid the wire electrode from causing disconnection from a stage in which the plate thickness of the workpiece rapidly changes to a stage where it changes gradually, further, in respect of an unpredictable change in the plate thickness the machining must be carried out by reducing the machining current and in that case, the machining speed is significantly retarded requiring the machining time period. Moreover, the amount of enlarging machining differs by the plate thickness as mentioned above, modification needs to carry out again.